Radiation shield



RADIATION SmLD No Drawing. Application January 29, 1947, Serial No.725,144

9 Claims. or. 252-478 This invention concerns a composition matter andprocess for shielding against radiation. More particularly, thisinvention concerns a plastic type shield which is useful for shieldingagainst the radiations encountered in an environment such as a pile typeneutronic reactor, and

which is eifective against both slow and fast neutrons but especiallyeffective against slow neutrons.

In the prior art, the problem of shielding against radiations has beenprimarily with respect to shielding against X-rays and the like. Thishas been accomplished usually by various lead containing materials.There have also been shielding problems in connection with cyclotronswhich have been handled by the use of large tanks of water.

In relatively recent years, however, there have been developed devicesdescribed as neutron reactors, specifically illustrated by piles,wherein fission of certain heavy isotopes such as U and Pu isaccomplished. As a result of this atomic disintegration not only aregreat amounts of heat and other forms of energy produced, but there areevolved radiations comprising slow and fast neutrons in addition to betaand gamma rays. Not only are these radiations of an intensity and energymany times greater than radiations heretofore encountered in the priorart, but in the instance of fast'and slow neutrons, the character of theradiation is considerably different than X- rays and similar radiationsheretofore encountered industrially on a large scale.

It is therefore apparent that because of the different and highenergy'factors involved that the development of shielding of the classindicated, particularly shielding effective against neutrons,constitutes a problem of considerable magnitude and one in whichpriorwork is not of great help;

After extensiveinvestigation we have found that there are certainplastic materials in which there may be incorporated certain inorganiccompounds to give a composition of matter that is an efiective anddurable shield against fast and slow neutrons. There-are a number ofmethods of employing this shielding, particularly in connection withcarrrying out biological work involving studies of the efiects ofirradiation or particle bombardment of living tissue.

This invention has for one object to provide shielding material.

Another object is to provide shielding material effective against fastand slow neutrons.

Still another object is to provide a shielding material that issubstantially free of elements which become dangerously radioactive as aresult of neutron bombardment.

A still further object is to provide a shielding material for use inenvironments of high neutron flux, in connection with biologicalstudies.

A still further object is to provide a shielding material that may befabricated into sheets, boxes, and other forms having high mechanicalstrength, durability, and radiation resistance.

Another object is to provide methods for manufacturingshielding of theclass described.

Other objects will appear hereinafter.

We have found that a very satisfactory shielding com position of mattermay be prepared by incorporating boron compounds in plastic material,the atomic composition of the plastic material being essentiallycomposed of hydrogen, carbon, and oxygen. More specifically, we havefound that the Various commercially obtainable polymeric plasticmaterials exemplified by the variousacrylate' type resins may besuitably impregnated or admixed with borax, boric acid, boric anhydride,or similar boron compounds which have the property of capturing neutronsefficiently, to obtain a shielding material of considerable utility. Theresulting compositions of matter possess suitable physicalcharacteristics to permit their;

fabrication into sheets, boxes, or various other structures as may bedesired. We have further found that because" such plastic compositionsare principally comprised of the elements carbon, hydrogen, and oxygen,shielding of this character is particularly useful in connection withbiological research. For example, such shielding does not containcomponents which would become objectionably radioactive with respect toinduced beta or gamma activity when subjected to high intensity neutronirradiation.

The preferred plastics for use in our shielding compositions comprisethe methyl methacrylate resins. The boron-methyl methacrylatecompositions of the present invention have been found to be surprisinglystable in intense radiation fields in spite of indications to thecontrary in the prior art literature, cf. Nature (London), vol. 143,page 640 (1939), and Proceedings of the Physical Society (London), vol.50, page 438 (1938). Any methyl methacrylate resin having the desiredmechanical properties may be employed in our compositions, and thecommer-' cially available resins of this type such as Lucite, Plex-.iglas, and Crystallite have been-found to be fully satis: factory.

Any boron compound may be employed in shielding prepared in accordancewith the present invention, but we prefer to use solid compounds havingthe mechanical characteristics of the pigments and fillers employedgenerally in molded plastics. We also prefer to employ boron compoundswhich have a high weight percentage of boron and are free from elementsin which objectionable beta or gamma radioactivity can be induced byneutron irradiation. Borax, boric acid, and boric anhydride meet theabove requirements satisfactorily, but boron carbide has been found tobe outstanding in all of these respects.

The concentration of boron compound to be employed in the shielding mayvary over a considerable range, depending on the neutron flux to beencountered and the neutron energies desired to be shielded out. Boronpreferentially absorbs slow neutrons, its absorption cross sectionbearing an approximately linear relationship to the reciprocal of thesquare root of the energy of the neutrons. Thus, the absorption crosssection of boron of natural isotopic constitution is about 1X10- sq. cm.for neutrons of about 20,000 e. v. and about 700x10- sq. cm. for thermalneutrons of about 0.03 e. v. It is therefore possible to estimate theoptimum quantity of boron to be employed in the shielding of the presentinvention for any particular application requiring neutron absorption.

The concentration of any particular boron compound in a shield of thepresent type may generally be considered in terms of units of weight ofthe boron content of the compound, per square unit of area of theshield, irrespective of the thickness of the shield. Concentrations ofboron ranging from 0.1 g. to 0.5 g. per sq. cm. of shield surface aregenerally satisfactory. Our preferred shielding contains approximately0.25 g. of boron per sq. cm.,

Patented June 18, 1957 which is sufiicient to reduce the intensity ofslow neutrons i pound may be chosen in accordance with the shieldthick-- ness andmechanical characteristics desired. When employing boroncarbide in concentrations of 0.30-0.32 g. per sq. cm., shields asthin as7 inch have adequate mechanical strength for most purposes. Somewhatthicker shields may be required when employing other compounds of lowerboron content at the same boron concentration per unit ofareaJ In anycase, the proportions may bevaried over a wide range without affectingthe shielding properties of the composition as long asthe concentrationof boron per unit area is maintained at the desired value. f I c I Theboron compound may be incorporated in the methyl methacrylate resin inaccordance with previous practices for the incorporation of pigments,fillers, and the likepin'to plastic compositions. A very hard boroncompound such as boron carbide may be irnbedded in the surface of arelatively hard plastic by pressure rolls or similar means. Mostconveniently, however, the boron compound is incorporated in partiallypolymerized methyl methacrylate and the mixture is then formed into thedesired shapes by casting, molding, or extruding. During or after theforming operation the material is subjected to further polymerization,for example by heating at a temperature of 7090 C., to obtain thedesired hardness.

Our invention will be further illustrated by the following specificexamples:

Example I Boron carbide powder was suspended in a viscous partiallypolymerized commercial methyl methacrylate resin. The resultingsuspension was cast between glass plates to form H inch sheet and wasthen hardened at 70-90 C. The resulting sheet had a boron carbideconcentration of approximately 0.31 g. per sq. cm. and was effective inreducing the intensity of a beam of thermal neutrons by a factor ofabout 10 Example II Commercially molded Plexiglas sheet of inchthickness, containing #400 grain boron carbide abrasive powder in aratio of lbs. per sq. ft. of sheet, was employed for the construction ofa biological exposure tunnel and other biological shields for use in aneutronic reactor. This shielding material showed no deterioration orobjectionably high induced radioactivity after more than 1000experimental exposures in a pile operating at power levelsof the orderof 4000 kw. It is to be understood that the above examples are merelyillustrative and do not limit the scope of our invention. Otherequivalent resins and boron compounds, or mixtures thereof, may besubstituted for the specific materials employed in these examples, andother shapes and thicknesses of shielding may be prepared in accordancewith the foregoing description. In general, it may be said that the useof any equivalents or modifications of procedure which would naturallyoccur to those skilled in the art is included in the scope of ourinvention. Only such limitations should be imposed on the scope of thisinvention as are indicated in the appended claims.

We claim:

1. The new and improved method for protecting an environment againstneutron radiation which comprises interposing between the source of saidradiation and said environment, as a shield, a mass of polymerizedacrylate plastic, having .incorporated'therein a sufficient quantity ofa boron compound to provide at least 0.1 gram of boron per sq. cm. ofshield area.

2. The new and improved method for protecting environment againstneutron radiation which comprises interposing between the source of saidradiation and said environment, as a shield, a mass of methylmethacrylate plastic containing a sufiicient quantity of a solid boroncompound to provide from 0.1 to 0.5 gram of boron per sq. cm. of shieldarea. I

3. The method of claim 2 in which the boron compound is boron carbide.

4. The method of claim 2 in which the boron compound isboricacid. V

5. The method of claim 2 in which the boron compound is boric anhydride.7 r 6. The new and improved method for protecting an environment againstneutron radiation which comprises interposing between the source of saidradiation and said environment, as a shield, a mass of methylmethacrylate plastic sheet, containing sutficient quantity of a boroncompound to provide approximately 0.25 gram of boron per sq. cm. ofsheet area.

7. The method of claim 6 in which the boron compound is boron carbide.

8. In a method for reducing the, neutron intensity in a protected areawithin a neutronic radiation field the improvement which comprisesinterposing, between the neutron source and said protected area, ashield comprising essentially a methyl methacrylate plastic havingincorporated therein a sufiicient quantity of a boron compound toprovide at least 0.1 gram of boron per square centimeter of shield area.

9. The method of claim 2 wherein the boron compound is borax.

References Cited in the file of this patent UNITED STATES PATENTS2,045,651 Hill June 30, 1936 2,400,477 Atwood May 21, 1946 2,439,374Leader et al. Apr. 13, 1948 2,462,270 7 Lipson Feb. 22, 1949 FOREIGNPATENTS 114,150 Australia May 2, 1940 861,390 France Oct. 28, 1940 OTHERREFERENCES Smyth, Atomic Energy for Military Purposes, August 1945, pp.28, 45, 85.

Perlrnan, Atomic Energy in Industry, U. S. A. E. C. Document No.MDDC-592 dated Nov. 18, 1946,declassified Jan. 15, 1947, 8 pages.

Review of Modern Physics, vol. 12, No. 1, January 1940, pages 14, 15,16, 17 of an article by Turner.

Sourcebook of Atomic Energy by Samuel Glas'stone, published by D. VanNostrand Co., N. Y., 1950, page 394.

Nature (London), vol. 143, page 640 (1939).

Smyth Report, pages 103 and 179.

1. THE NEW AND IMPROVED METHOD FOR PROTECTING AN ENVIRONMENT AGAINSTNEUTRON RADIATION WHICH COMPRISES INTERPOSING BETWEEN THE SOURCE OF SAIDRADIATION AND SAID ENVIRONMENT, AS A SHIELD, A MASS OF POLYMERIZEDACRYLATE PLASTIC, HAVING INCORPORATED THEREIN A SUFFICIENT QUANTITY OF ABORON COMPOUND TO PROVIDE AT LEAST 0.1 GRAM OF BORON PER SQ. CM. OFSHIELD AREA.